Same happened to me the other month Ian.
Sent cheques off to the Coop Bank, didnt appear on the statement.
In the meantime I sent off some more, they appeared on the statement.
Queried the "missing" ones with the bank, there ws no way they could be in their system and not allocated - assuming I filled in the details correctly, which I had.
Wrote to the clients involved, stated getting "repeat" cheques, and lo and hehold, the missing cheques appeared. Had to contact the clients again, very embarrassing.
Somethings have to get lost in the postal system I suppose, millions of letters each day.
Its happened twice in about 15 years with my bank, not their fault, and not a bad record for the Post Office.